This invention relates generally to the development of electrostatic images, and more particularly concerns a development apparatus having developer housing design with improved sump mass variation latitude.
Generally, the process of electrophotographic printing includes sensitizing a photoconductive surface by charging it to a substantially uniform potential. The charge is selectively dissipated in accordance with a pattern of activating radiation corresponding to a desired image. The selective dissipation of the charge leaves a latent charge pattern that is developed by bringing a developer material into contact therewith. This process forms a toner powder image on the photoconductive surface which is subsequently transferred to a copy sheet. Finally, the powder image is heated to permanently affix it to the copy sheet in image configuration.
Two component and single component developer materials are commonly used. A typical two component developer material comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles having an electrostatic charge so that they will be attracted to, and adhere to, the latent image on the photoconductive surface.
There are various known development systems for bringing toner particles to a latent image on a photoconductive surface. Single component development systems use a donor roll for transporting charged toner to the development nip defined by the donor roll and the photoconductive surface. The toner is developed on the latent image recorded on the photoconductive surface by a combination of mechanical scavengeless development. A scavengeless development system uses a donor roll with a plurality of electrode wires closely spaced therefrom in the development zone. An AC voltage is applied to the wires detaching the toner from the donor roll and forming a toner powder cloud in the development zone. The electrostatic fields generated by the latent image attract toner from the toner cloud to develop the latent image.
In another type of scavengeless system, a magnetic developer roll attracts developer from a reservoir. The developer includes carrier and toner. The toner is attracted from the carrier to a donor roll. The donor roll then carries the toner into proximity with the latent image.
Two component developer housings need a uniform layer of developer material on the developer roll. This must be accomplished independent of sump mass variations, machine tilt, toner concentration, environmental conditions, etc. Traditionally, this means designing the magnetic “pick-up” function to acquire much more developer material than needed and then trimming off the excess. This approach does provide sufficient material to form a “full brush” over a limited range of adverse conditions. However, under nominal conditions, perhaps as much as 80% of the material initially acquired is trimmed off. This increases developer roll surface wear (because of the generally required very strong magnetic “pick-up” field), reduces developer material life, and wastes energy.
It is desirable to have a developer housing design that provides a constant flow (quantity) of developer material to the development nip region of a magnetic roll over a wide range of developer sump mass (volumes) (wider than traditional approaches).